How to fix blackberry bold 9900 loop?

Hello, recently my blackberry fell into water, atfer that it didnt switch of, there was red led light and some noise coming from it. After that, I've downgraded its OS to the oldest I could and now its switching on, going thourh boot screen and showing main screen for couple of seconds and/or starts to show demo, atfer that it restarts.
When i tried newer OS it didnt get even that far, but started again red light and noise, and nothing. Is there something i can do?

First, your BB fell into water. You didn't mention how soon after you've attempted resuscitation of the device. Turning off the device, removing the casing/battery/SIM/SD card and not even attempting a power up of the device until it is completely dry by simply leaving it on a shelf (or using desiccant/silica gel/rice/dehumidifier/fan - opinions vary) is the recommendation. Failure to do so can result in circuits shorting out and significant damage to the device. Even so, corrosion remains a possibility.

Second, you've added a significant change to the device in attempting to restore an older OS - and during a period when the device is already compromised. So now it's more difficult to determine the root cause.

Noise. The crackles of circuits frying? Muffled speaker from moisture? Difficult to say.

In any case, the damage may have already been done but I would allow the phone to simply sit for a few days, powered off. When you're confident it's absolutely dry then attempt an autoloader on the device with the OS of your choosing. An older OS isn't going to make revival easier so you may as well choose the latest to your liking.